1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus with a thin-film magnetic head having microwave magnetic exciting function for recording data signal onto a magnetic recording medium that has a large coercivity for thermally stabilizing the magnetization.
2. Description of the Related Art
With the demand for higher recording density of a magnetic recording and reproducing apparatus such as a magnetic disk drive apparatus, each bit cell in a magnetic recording medium for recording digital information has been miniaturized and, as a result, a signal detected by a read head element in the thin-film magnetic head sways due to such as thermal fluctuation. This causes deterioration in a signal-to-noise ratio (S/N), and in the worst case, the signal detected by the read head element may disappear.
It is effective for a magnetic recording medium adopted for the perpendicular magnetic recording scheme that is recently put to practical use to increase perpendicular magnetic anisotropy energy Ku of a magnetic recording layer in this recording medium. On the other hand, a thermal stabilization factor S that corresponds to the thermal fluctuation is represented by the following equation (1) and is necessary to have in general 50 or more:S=Ku·V/KB·T   (1)where Ku is perpendicular magnetic anisotropy energy, V is a volume of crystal grains that form the recording layer, kB is the Boltzmann constant, and T is an absolute temperature.
According to the so-called Stoner-Wohlfarth model, an anisotropy magnetic field Hk and a coercivity Hc of the recording layer is represented as the following equation (2):Hk=Hc=2Ku/Ms   (2)where Ms is a saturated magnetization of the recording layer.
The coercivity Hc increase with the increase in the perpendicular magnetic anisotropy energy Ku. In a normal recording layer, however, Hk is higher than Hc.
In order to perform desired inversion of magnetization in the magnetic recording layer in response to data sequence to be written, a write head element of the thin-film magnetic head is required to apply a recording magnetic field having a precipitous rising edge and a level up to about the anisotropy magnetic field Hk of the recording layer. In a hard disk drive (HDD) apparatus adopting the perpendicular magnetic recording scheme, a write head element with a single pole is used so that a recording magnetic field is applied perpendicular to the recording layer from an air-bearing surface (ABS) of the element. Since an intensity of this perpendicular recording magnetic field is proportional to a saturated magnetic flux density Bs of the soft magnetic material that forms the single pole, a material with a saturated magnetic flux density Bs as high as possible is developed and is put into practical use for the single pole. However, the saturated magnetic flux density Bs has the practical upper limit of Bs=2.4 T (tesla) from a so-called Slater-Pauling curve, and a recent value of the saturated magnetic flux density Bs of soft magnetic material closes to this practical upper limit. Also, in order to increase the recording density, the thickness and width of the single pole have to decrease from the present thickness and width of about 100-200 nm causing the perpendicular magnetic field produced from the single pole to more lower.
As aforementioned, due to the limit of recording ability of the write head element, high-density recording becomes difficult now. To overcome such problems, suggested is so-called thermal assisted magnetic recording (TAMR) scheme for recording a magnetic signal on a recording layer of the magnetic recording medium under conditions where the recording layer is irradiated by a laser beam for example to increase the temperature and to lower the coercivity Hc of the magnetic recording layer.
Japanese patent publication No. 2001-250201 discloses a TAMR technique in which electrons are radiated to a magnetic recording medium from an electron radiation source to heat a recording part in the magnetic recording medium so that the coercivity Hc is lowered and thus it is possible to record magnetic information on the medium using a magnetic write head.
U.S. Pat. No. 7,133,230 B2 discloses another TAMR technique in which a laser beam from a semiconductor laser element formed in a perpendicular magnetic recording head is irradiated to a scattering member or near-field light probe formed in contact with a main pole of the head so as to produce a near-field light, and the produced near-field light is applied to the magnetic recording medium to heat it and rise the temperature.
However, there are various difficulties and problems in these TAMR techniques. For example, (1) a structure of the thin-film magnetic head becomes extremely complicated and its manufacturing cost becomes expensive because the head has to have both a magnetic element and an optical element, (2) it is required to develop a magnetic recording layer with a coercivity Hc of high temperature-dependency, (3) adjacent track erase or unstable recording state may occur due to thermal demagnetization during the recording process.
Recently, in order to increase sensitivity of a giant magnetoresistive effect (GMR) read head element or a tunnel magnetoresistive effect (TMR) read head element, study of spin transfer in electron conductivity is made active.
US patent publication No. 2007/0253106 A1 and J. Zhu, “Recording Well Below Medium Coercivity Assisted by Localized Microwave Utilizing Spin Transfer”, Digest of MMM, 2005 disclose application of this spin transfer technique to the inversion of magnetization in a recording layer of a magnetic recording medium so as to reduce a perpendicular magnetic field necessary for the magnetization inversion.
According to this scheme, an alternating magnetic field of high frequency is applied to the magnetic recording medium in a direction parallel to its surface together with the perpendicular recording magnetic field. The frequency of the alternating in-plane magnetic field applied to the magnetic recording medium is an extremely high frequency in the microwave frequency band such as several GHz to 10 GHz, which corresponds to a ferromagnetic resonance frequency of the recording layer. It is reported that, as a result of simultaneous application of the alternating in-plane magnetic field and the perpendicular recording magnetic field to the magnetic recording medium, a perpendicular magnetic field necessary for the magnetization inversion can be reduced to about 60% of the anisotropy magnetic field Hk of the recording layer. If this scheme is put in practical use, it is possible to increase the anisotropy magnetic field Hk of the recording layer and thus it is expected to greatly improve the magnetic recording density without utilizing the complicated TAMR system.
However, according to this magnetically assisted magnetic recording scheme wherein the alternating in-plane magnetic field and the perpendicular recording magnetic field are simultaneously applied to the magnetic recording medium, since a microwave frequency band signal or microwave excitation signal of several GHz to 10 GHz is transmitted to a thin-film magnetic head, microwave excitation energy may be radiated in a housing of the magnetic disk drive apparatus from the thin-film magnetic head or from the transmission line of the microwave signal, and may be propagated there through.
If the microwave excitation energy is radiated in the housing of the magnetic disk drive apparatus from the thin-film magnetic head or from its transmission line, a standing wave with peaks and valleys may occur in the housing. Due to the peaks and valleys of the standing wave, it is impossible to keep the electrical potential on the magnetic recording medium constant. Also, if a frequency resonance occurs at a part of the housing depending upon the dimension of space, partial absorption of energy may occur causing the microwave excitation energy to become unstable.